Particle classification processes are of critical importance for many grinding circuits. In general, the energy consumption for grinding circuits can be reduced drastically as classification efficiency is high. Ideal classification can be defined as the separation of particle stream into two frictions, one containing only fine material (below a specified size) and the second of only coarse particles. However, in practice, some grains of the feed to the classifier can be taken both to the coarse and to the fine stream. The contents of fine material in the coarse stream and coarse material in the fine stream gives a measure of classification efficiency or classification sharpness. The amount of coarse material in the fine fraction is determined mainly by rotor construction. In practice, rotor classifiers (forced vortex) provide very low contens of coarse material in the fine stream. On the other hand, the amount of fine material in the coarse stream is very different for many classifiers as it depends on its construction. This is the main indicator of the classification efficiency.
U.S. Pat. No. 4,260,478 discloses an apparatus for classifying particles comprising a body having a fine particle outlet at the top of the apparatus and a coarse particle outlet at the bottom. An air flow comprising dispersed unclassified material is supplied to the classification zone from below through a vertically arranged supply pipe. The classification zone is provided with two co-axially arranged rotors or separating wheels, in which the inner rotor is provided with a feed cone which co-rotates with the same. The feed cone is arranged to disperse the material into the classification zone. This construction has several disadvantages; first, the feed cone is subject to high wear due to high impact force between the rotating feed cone and the upward flowing unclassified material. After a certain time of operation, the cone may exhibit un-evenly distributed grooves and similar in the external surface of the same, which may create unbalance and wear of the rotor blade bearings and the engine connected with the same, and second, the outlet section for removal of air and fine classified material from the classification zone is shaped as an ordinary 90.degree. bend, which result in a friction loss and for that reason higher energy consumption at the air supply end of the classification process. Moreover, as the largest diameter of the feed cone is smaller than the lower diameter of the inner rotor blades, material will contact the rotor blades and result in a wear of the same.
A similar apparatus is disclosed in U.S. Pat. No. 2,968,401. Here, there is no air supply pipe arranged vertically within the classification zone, and the material to be classified is supplied directly into the classification zone dispersed in an air flow. The latter construction produces low classification efficiency as fine material is mixed with unclassified material. As with the first mentioned prior art construction, the feed cone is fixed to the rotor blades and co-rotates with the same. Moreover, the single point radial supply of material to be classified provides a poor degree of dispersion. This construction, however, is provided with a spiral shaped oulet for fine material, which reduces the friction loss as the centrifugal force is converted to unidirectional kinetic energy.
DE Patent No. 920.704 discloses a particle separator of the similar type as described above. This construction, however, represents an early stage in the development of such apparatuses, and produces a poor classification efficiency, mainly due to the small volume available to classification. Also in this construction, the rotor blades are subject to wear as the particle feed contacts the blades directly.
The closes prior art is considered to be represented by U.S. Pat. No. 4,528,091, which is the preferred construction in commercial utilization, particularly with regard to classification efficiency.
The classification zone is provided with four rotor units distributed in a horisontal plane in an equal mutual distance, and the rotors rotates about a horisontal axis. Here, a stationary feed cone is provided below the rotors. A vertically arranged supply pipe for partly classified material, from the secondary classification zone below, dispersed in an air flow is located beneath the feed cone. The supply pipe exhibits a truncated cone shaped upper section and a sylindrically shaped lower section which terminates above a secondary classification zone. The secondary classification zone is supplied with air flowing tagentially into the same, and is provided with a further rotor unit arranged coaxially with the longitudinal axis of the main apparatus.
Material to be classified is supplied to the classification zone with a screw conveyor into the annular section establised by the internal casing of the apparatus and the external surface of the supply pipe.
Also this construction has several disadvantages. First, the peripheral supply of material feed results in a poor dispersion of the particulate material in the air, which again results in a lower classification efficiency, and second, the arrangement of the secondary classification zone will establish a stationary (non-rotating) zone at the axis of rotation of air and dispersed particulate material, which further decreases the classification efficiency.
Accordingly, there is a need in the art for an apparatus which, in addition to a high classification efficiency, provides a low operation cost as compared with the existing technical solutions.